Apparatus for decoding a stereophonic pair of left and right input audio signals into a multiplicity of output signals is commonly referred to as a surround sound decoder or processor. Surround sound decoders work by combining the left and right input audio signals in different proportions to produce the multiplicity N of output signals. The various combinations of the input audio signals may be mathematically described in terms of a N row by 2 column matrix, in which there are 2N coefficients each relating the proportion of either left or right input audio signals contained in a particular output signal.
The matrix coefficients may be fixed, in which case the matrix is called passive, or they may vary in time in a manner defined by one or more control signals, in which case the matrix is described as active. The coefficients in a decoding matrix may be real or complex. Complex coefficients in practice involve the use of precise phase quadrature networks, which are expensive, and therefore most recent surround sound decoders do not include them, so that all of the matrix coefficients are real. In the bulk of the work described in this patent application, the matrix elements are also real. Real coefficients are inexpensive and will optimally decode a five channel film encoded with the active encoder described in this patent.
However, real coefficients are not optimal when decoding a film encoded from a five channel original using a passive encoder such as the one described in this application, and are also not optimal when decoding a film made with the standard four channel encoder of the prior art. A modification to the decoder design which will optimally decode such films is also described. Although the description is of a phase corrector to the inputs of the decoder, the correction could also be accomplished by making the matrix elements complex.
In a passive matrix, which is defined as a matrix in which the coefficients are constant, such as the Dolby Surround matrix, several ideal properties are achieved by suitable choice of the coefficients. These properties include the following:
Signals encoded with a standard encoder will be reproduced by a passive matrix decoder with equal loudness regardless of their encoded direction.
Signals where there is no specific encoded direction, such as music that has been recorded so that the two inputs to the decoder have no correlation, that is, decorrelated signals, will be reproduced with equal loudness in all output channels.
When the input signals are a combination of a directionally encoded component and a decorrelated component there is no change in either the loudness or the apparent separation of the decorrelated component as the encoded direction of the directionally encoded component changes.
A disadvantage of passive decoders is that the separation of both directional and decorrelated components of the input signals is not optimal. For example, a signal intended to come from front center is also reproduced in the left and right front output channels usually with a level difference of only 3 dB. Therefore, most modern decoders employ some variation of the matrix coefficients with the apparent direction of the predominant sound source, that is, they are active rather than passive.
In the original Dolby Surround decoder format, only one rear channel output is provided, which typically is reproduced on more than one loudspeaker, all such loudspeakers being driven in parallel, so that there is no left-right separation in the rear channels. However, there is high separation between signals that are encoded in opposite directions.
Previous patents have described many aspects of active matrix surround sound decoders for conversion of a stereophonic audio signal pair into multiple output signals. The prior art describes how the apparent direction of a directionally encoded signal component can be determined from the logarithm of the ratio of the amplitudes of the component in the left and right channels of the stereophonic pair, along with the logarithm of the ratio between the sum of these amplitudes and the difference therebetween. This art will be assumed in this patent application, along with a great deal of art which pertains to smoothing the directional control signals thus or otherwise derived. We assume that these two directional control signals exist in a usable form. For the purposes of this invention, these directional control signals can be possibly derived from directional information recorded on a subchannel of a digital audio signal.
This invention concerns the use to which these directional control signals are put in controlling an active matrix which takes the signals on the two inputs and distributes them to a number of output channels in appropriately varying proportions dependent upon the directional control signals.
A simple example of such a matrix is given by Scheiber in U.S. Pat. No. 3,959,590. Another matrix in common use is that of Mandell, described in U.S. Pat. No. 5,046,098. A matrix with four outputs is described in detail in Greisinger, U.S. Pat. No. 4,862,502, and a complete mathematical description of this matrix, along with a mathematical description of a six output matrix, is given in Greisinger, U.S. Pat. No. 5,136,650. A different six output matrix is described in Fosgate, U.S. Pat. No. 5,307,415. All of these prior matrices distribute the input audio signals among the various outputs under control of the directional control signals as described above.
Each of these matrices is constructed somewhat differently, but in each case each output is formed by a sum of the two input signals, each input signal having been first multiplied by a coefficient. Thus each matrix in the prior art can be completely specified by knowing the value of two coefficients for each output and how these coefficients vary as a function of the directional control signals which provide directional information as described above. These two coefficients are the matrix elements of a N by 2 matrix, where N is the number of output channels, which completely specifies the character of the decoder. In most prior art these matrix elements are not explicitly stated, but can be inferred from the descriptions given. In a particular embodiment they can also be easily measured.
Greisinger, U.S. Pat. No. 5,136.650, issued Aug. 4, 1992, gives the complete functional dependence of each matrix element on the directional control signals.
Since the above-referenced Greisinger patent issued, the film industry has developed a "five plus one" discrete sound standard. Many theater movie releases and some home releases are made with soundtracks comprising five separate full bandwidth audio channels, namely center, left front, right front, left rear, and right rear, with a reduced bandwidth sixth audio channel intended for very low frequency effects. Reproduction of such soundtracks requires special digital hardware to demultiplex and decompress the audio tracks into the 5+1 output channels. However there is a very large selection of previously released film prints and videos which employ a two channel soundtrack matrix encoded format, both analog and digital. Such soundtracks are encoded during the mixing process using a standardized four channel to two channel encoder.
While earlier work by Greisinger and others has described the outputs of the decoder in terms of a complicated sum of various signals: the input signals, their sum and their difference, and the same four signals after passing through variable gain amplifiers controlled by the directional control signals, it is possible to collect the terms of each output that are related to a particular input and thereby to describe the matrix completely in closed form, so that the decoder can be realized either in digital or analog hardware components.
The standard encoder for two channel soundtrack matrix encoding has limitations, and an improved passive encoder or an active encoder can be used to generate two channel matrix encoded soundtracks that achieve better performance when decoded through a surround sound decoder according to the invention.
In a standard film decoder, a boost is applied to the front channels when a strongly steered signal such as dialog is present. This upsets the balance between such signals and background effects or music, relative to the balance between such signals in the discrete 5 channel movie theater system. An improved active encoder described herein is needed to correct the balance between the strongly steered front signals and music. There is also a need to improve both encoder and decoder performance in regard to left side and right side signals. A further improvement in the decoder is to limit the effects of abrupt changes in the directional control signals to provide better dynamic response to rapid changes therein.